The space shuttle uses dedicated signal conditioners (DSCs) for the purpose of conditioning transducer outputs and other signals to make them compatible with orbiter telemetry, displays and data processing systems. The dedicated signal conditioners are located throughout the orbiter, often in difficult to access locations in the vehicle fuselage. When troubleshooting a potential instrumentation problem, personnel frequently have to demate cables to verify that the cables are not the source of the problem. However, once a cable is demated, all dedicated signal conditioners and other systems which have a wire passing through the cable's connector have to be re-tested after the cable is re-connected. Due to their inaccessibility, the dedicated signal conditioners sometimes have to be removed in order to check them. This results in many man hours of re-validation testing on systems that were unrelated to the original problem. The cost to the shuttle program for these re-testing procedures is exorbitant. A system which allows cable continuity to be checked non-intrusively without demating the cables would therefore save many hours of testing and substantially reduce testing costs.
One known technique for non-intrusively checking cable continuity is time domain reflectometry (TDR). In time domain reflectometry, a high frequency pulse is injected into one end of the cable. A discontinuity in the cable causes a reflection of the pulse that can be detected back at the end where the pulse was injected. The location of this discontinuity can be determined by measuring the time interval between when the pulse was injected into the cable, and when the reflected pulse is detected. Also, the polarity of the reflected pulse is indicative of whether the discontinuity is a short circuit or an open circuit.
Unfortunately, time domain reflectometry is not feasible for detecting cable faults in the space shuttle. This is because time domain reflectometry uses high frequencies which are above a frequency cut-off of the dedicated signal conditioners. Further, the faults are typically only a few meters or less from the injection point, and time domain reflectometry is ineffective at these distances due to the extreme short total travel time (e.g., a few nanoseconds) of high frequency pulse.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for non-intrusive cable testing technique which provides the accuracy necessary for use in the space shuttle environment, and the like.